In general, in a focal plane shutter wherein shutter curtains of flexible material such as cloth are wound on a drum, the drum makes several 360.degree. rotations for one run of the shutter and the rotation of the drum is converted into rotation of a speed change gear which rotates less than 360.degree. rotations for each shutter operation, so that the winding operation and the shutter running operation are controlled by the speed change gear.
The left half of FIG. 1 is a sectional view showing one example of a conventional speed change gear supporting mechanism for such a shutter mechanism. A winding gear 3, a top curtain speed change gear 4 and a bottom curtain speed change gear 5 are rotatably mounted on a speed change gear shaft 2 which is fixedly secured to a camera body 1. A winding pin 3a is provided on the lower surface of the winding gear 3 and the pin 3a is engaged with a pin 4a embedded in the upper surface of the top curtain speed change gear 4. A pin 4b is provided on the lower surface of the top curtain speed change gear 4 and it is engaged with a pin 5a provided on the upper surface of the bottom curtain speed change gear 5. Therefore, as the winding gear 3 is turned, the top curtain speed change gear 4 and the bottom curtain speed change gear 5 are turned through the engagement of the pins.
In the supporting mechanism shown on the left side of FIG. 1, the gears 3, 4 and 5 are slidably turned around the speed change gear shaft 2, and the winding force is transmitted through the engaged pins. Due to the stability of the slide bearing between each gear and the shaft 2, the degree of vibration of each gear is low, and the shutter mechanism is stable. However, the bearing portions are liable to be damaged, which increases the frictional loss. Since this frictional loss is added to the shutter charging force, a large force is required for winding the shutter.
These difficulties may be overcome by inserting a rolling element between the speed change gear shaft and each of the gears as shown in the right half of FIG. 1. While it is true that the friction loss of the supporting mechanism is smaller than that of the supporting mechanism shown in the left half of FIG. 1, in many cases it is difficult to provide the rolling elements in two rows, i.e. upper and lower rows for each of the gears, because of the limit in the axial dimensions of each gear, and therefore only one row of rolling elements are usually provided for each gear near the center thereof in the axial direction. Accordingly, if the portion of each gear supporting the rolling elements is not accurately fitted, then the degree of vibration of the gear is increased. In such a case it is necessary to increase the lengths of the abutting pins and the margins between the gears in the axial direction. Furthermore, since the surfaces of the top curtain speed change gear 4 and the bottom curtain speed change gear 5 vibrate considerably, it is difficult to accurately set members for locking the speed change gears 4 and 5, which makes the shutter speed unstable.
In the case of an electrical shutter, the initial operation of the top curtain speed change gear 4, which is effected by the release operation after the winding operation has been accomplished, operates a timing switch. Since the gear 4 vibrates excessively, it is difficult to set the switching time of the timing switch, and this will also make the shutter speed unstable.